The effect of mutagen treatment on the immunogenic and metastatic properties of tumor cells was investigated. After two courses of in vitro treatment of 3LL tumor cells with UV light (254 nm) or B16BL6 melanoma cells with MNNG, highly immunogenic cell variants were obtained. These immunogenic variants of 3LL or BL6 tumor cells were rejected in the immunocompetent C57BL/6 mice, in all irradiated (55OR) or athymic nude mice. All 15 clones selected from the parental tumors were able to grow in the immunocompetent mice. Some clones from the mutagen-treated tumor cells (12 out of 80 clones of 3LL and 34 of 48 clones of BL672 tumor) were rejected in 60-100% of inoculated immunocompetent mice, but grew in nude mice (tum- clones). Mice which were able to reject the first tumor inoculum were resistant to subsequent challenge with high doses of tumor cells. Immunogenic variants of 3LL tumor showed complete cross protection when immune mice were challenged with nonidentical tum- clones, as well as tum+ clones or the parental 3LL tumor cells. Mutagen treatment of 3LL and BL6 tumor cells not only increased their immunogenicity but also reduced their metastatic ability. Using monoclonal antibodies and flow cytometry analyses, the expression of H-2 antigens on the cell surface of the immunogenic and nonimmunogenic tumor cell variants was investigated. A substantial increase in the expression of H-2Kb and H-2Db antigen in the BL6T2 cells and its clone was found. Tum- clones of 3LL cells also had higher level of H-2 expression than tum+ cells. These date indicate that treatment with UV light or MNNG can be efficient in the increase the immunogenicity of tumor cells. The crossreactivity of the immunogenic variants with the parental tumors can be utilized for immunotherapy of the local or metastatic tumors in experimental animals.